The authors conducted a simulation experiment on an open source software. The paper mostly discusses about the difference in terms of data and expect results between Test-Driven Development and a non-Test-Driven Development process and how they would simulate them. In order to calibrate the model parameters, they used real project data obtained from another case study. In order to simulate TDD, they had to change a few parameters. First they had to increase the average time needed to write a line of production code since they needed to include the time needed to write test code. They also decreased the number of defects injected during coding activities based on the fact that doing tests before coding increase the probability of finding defects earlier. And the debugging time to fix a single defect will decrease. By running the simulations, they found that TDD produces better quality code than non-TDD processes.